Yeast was identified as the cause of fermentation in the 1800s, but beer has been around since long before then. How was fermentation started before the intentional introduction of yeast? Was it all lambic-style ambient yeast, or what?
You don't need to know what something is to use it effectively. Though yeast was only identified as a microorganism recently, it has been known as the cause of fermentation for many centuries. It's easy to underestimate how sophisticated people throughout history were.
Before yeast was monocultured in labs it was actively cultured by brewers. They would transfer the yeast cake from a batch that's finishing into a recently-brewed batch. This cake contained a whole ecosystem of yeasts and bacterias, but a few strains were usually dominant.
By applying artificial selection brewers developed different strains of yeast in different areas. The yeast culture was the make-or-break characteristic of a brewery, and defined the styles of beer that could be produced more than any other factor (other than water in Burton).
Evidence of this process comes from the history of lager: bottom-fermenting yeast (Saccharomyces carlsbergensis) emerged in the fifteenth century as a direct result of the cold storage regime of northern brewers such as Carlsberg.
They may not have known it was a microorganism, but middle ages brewers most certainly knew that yeast was the cause of fermentation. That's why they called it "Godisgood".
Actually, there are still people who brew the same way people did before the discovery of yeast, so we have a pretty good idea how it worked. There are also data from early 20-th century ethnological surveys where brewers describe their methods. Some of the equipment used for yeast transfer is still in use, or in ethnological museums.
People usually didn't brew that often, so they couldn't transfer the yeast cake itself. Instead, they would collect yeast from the previous brew and either keep it in a jar that they kept cool, for example in a well, or by drying it on a wooden log, piece of linen, straw ring, etc etc etc. There were lots of methods, but they basically boil down to either keeping wet or dry yeast. Odd Nordland's 1969 book is the only detailed source of information on this that I know; unfortunately it's near-impossible to get hold of.
As far as I know, this type of yeast and yeast use only survives in two places: the Lithuanian province of Aukstatija, and western Norway (specfically, Hardanger, Voss, and Sunnmøre).
I've written a summary of what's known of Norwegian practices, and earlier this year I was able to brew with a brewer who still uses his family's ancestral yeast strain. I've published an account of that brewing session.
I hope people will forgive me for linking to my own stuff here. I don't know of any other detailed sources of information on the web anywhere.
There are many plants where yeast grows in symbiotic mutuality. Take fresh dark grapes - the slight white "sediment" on the surface is natural yeast, and you won't find grapes without it.
Another such plant is barley. Normally, the yeast only appears on the seeds, in relatively small amounts. Malting creates optimal environment for the yeast growth though, making the whole volume of the seed (as opposed to just the surface) nutritious for the yeast, and as it grows, it produces alcohol.
Sure there is always a risk that a different culture of fungi or bacteria takes over, killing off the yeast (and e.g. producing vinegar instead of alcohol) but if brewing is performed in relative cleanliness, with little contaminants that could disrupt the process, the natural yeast will take over and dominate it, killing off all competing cultures.
Currently, cultured yeast provide a kick-start to the process, simply leaving no time for different cultures to dominate the batch, and radically shortening the process that would normally take a long time until natural yeast reaches concentrations you create by just adding cultured yeast, but it's not essential to the process - it's just a strong push in the right direction.
Before brewers discovered the importance of yeast in the brewing process they had to rely on the local wild yeasts for fermentation. Louis Pasteur discovered the importance of yeast to brewing in 1857.
Ancient brewers still used the same process of mashing the grains to extract sugars for fermentation and adding hops for bittering and preservation. As a side note, prior to the use of hops in brewing brewers used a mix of herbs called "gruit" which provided flavoring but no preservation.
Certain areas of the world are known to have wild yeasts better suited to brewing and still produce beers of this style today, such as Belgian lambics.
Yeast as a microorganism was discovered by Pasteur. The Vikings did not have microscopes, likewise neither did the Germains who wrote the beer purity law of 1516 which does not include yeast as in ingredient. Brewers (often clergy or women called "brewives") believed it was a mystical process and kept beer consistent by saving sediment in patties or cakes for the next batch like bakers save chunks of dough. The term yeast may have been used before pasteur but he surely changed the meanining and revolutionized standardized beers by isolating yeast types. As far as I know beers that rely on wild yeasts are the "farmhouse" styles compared to the main two; lager and ale yeasts.
There is no production of alcohol without yeast. Before the "discovery" (better: selection and cultivation) of the Yeasts we know and use today, humans had to rely on luck.
Its a wild fermentation, many different microbes and fungi are competing for their nutrition (sugar). The problem here is: Many yeasts produces fusel alcohol, i.e. methyl alcohol (which can cause blindness). Other bacteria could cause the fermentation to vinegar.
So the resulting product seldom was good, mostly it was not drinkable for us today ;)
Since the cultivation of special yeasts it's possible to control the fermentation, while adding a big amount of the yeast. So this yeast has a big advantage over others. Further the produced alcohol and carbondioxyde protects the beverage from other organisms.
The Vikings used to brew beer (some of us still do!). They did this without the specific knowledge of yeast. Each family owned a stock of wood that belonged to the family brewery which was used to stir the wort prior to fermentation. This inoculated the wort and provided the yeast needed for fermentation.
Although they didn't know why it happened, they did know that unless they used the stirring stock, they wouldn't get beer.